Many pipelines installed underwater are manufactured from synthetic materials, such as high-density polyethylene (HDPE) because of the superior corrosion resistance, and in certain applications, the superior wear resistance of synthetics over iron alloys. Synthetic pipelines are used in a wide variety of tasks for both industrial and municipal applications. As the depths of the installations and the lengths of the synthetic pipelines are increasing, better methods of installations must be developed.
Unlike pipelines manufactured from iron alloys, pipelines manufactured from synthetic materials do not possess the high tensile and compressive strengths associated with iron alloys. Unlike pipelines manufactured from iron alloys, synthetic pipelines possess a relative density which is close to, or less than, that of water. HDPE for example, has a relative density which is less than water, and consequently will float when placed in water. Because the relative density of synthetics is close to that of water they require that large amounts of additional weight, in the form of ballast, be attached to these pipelines to allow them to sink below the surface and to anchor them firmly upon the sea bed. Ballast weight that is attached prior to the installation of the pipeline is the most common type of ballast weighting. Often this ballast weighting consists of pre-cast concrete blocks attached to the pipeline with bolts, through openings preformed in the concrete blocks for this purpose.
There are specific problems encountered when installing underwater pipelines, arising from the relationship of the net submerged weight of the pipeline, that is the sum total weight of all of the components making up the pipeline, and the material strength of the pipeline itself This problem is compounded for pipelines produced from synthetic materials, as the lack of tensile and compressive strength of synthetic materials can make these pipelines prone to buckling from the forces exerted by the net submerged weight during the sinking process. While this invention aims to enhance the ease of installing synthetic pipelines it may also be applied to pipelines manufactured from metal or other compounds.
The amount of ballast weight attached to pipelines varies with the design of the pipeline and such factors as sub sea terrain, ocean currents, wave action and the type of product or substance the pipeline is designed to carry. A pipeline designed to carry a gas, will require a greater amount of ballast weight than a pipeline designed to carry products such as slurry, which has a relative density greater than water. Strong littoral currents or wave induced forces may also dictate that additional ballast weighting be applied to securely anchor the pipeline in specific sections. The amount of ballast weighting installed on the pipeline may not be distributed equally as a function of its length, as the forces of waves or current acting upon the pipeline may vary depending upon such factors as the variations of the depth and the length of the pipeline. Wave induced forces, acting on the pipeline, will generally decrease as the depth of water increases. Littoral currents, acting on the pipeline, will generally increase as the distance from shore becomes greater. Variations of the pipeline elevation due to the sub sea terrain, on which it is laid, may also dictate that ballast weighting of specific sections must also be increased or decreased for a specific section.
The ballast weighting of pipelines forms a significant portion of the total economic value of the pipeline. The amount of ballast weighting influences the method of launching the pipeline from its point of construction as well as the method of sinking the pipeline to the sea bed, both of which can be translated into economic costs.
The amount of weight added as ballast to a pipeline is commonly referred to as the offset weighting and is expressed as a ratio of the amount of ballast weight required to offset the buoyant force of a pipeline assumed to be partially or totally filled with air at atmospheric pressure. As an example, an offset weight requirement of 50% means that the ballast weight added to the pipeline negates the buoyant force of the pipeline if it were filled to 50% capacity with air. As another example, an offset weight requirement of 100% would mean that the ballast weight added to the pipeline would negate the buoyant force of the pipe if it were possible to fill the pipeline with air to 110% of it""s volume. Filling a pipeline to 110% of its volume is not possible although the practice of expressing the weight requirement in this way gives an exact indication of the amount of ballast weight required in relation to the size of the pipeline.
Offset weight requirements in excess of 95% will generally require that auxiliary buoyancy, in the form of floats, vessels etc., be temporarily attached to the pipeline, to allow it to float upon the body of water prior to its placement on the bed of the body of water. In lieu of using auxiliary buoyancy the pipeline designer may elect to remove some of the ballast weight, prior to the pipeline installation, and install the deleted ballast weight after installation.
As previously mentioned, it is beneficial to the design of pipelines to vary the amounts of ballast weight applied to specific sections of the pipeline. These sections may be identified as a section of the pipeline located from a specific datum or reference point. This variation of ballast weight, on these sections of the pipeline, can increase or decrease. As an example of changing ballast weighting, a hypothetical pipeline of 10,000 feet in length terminating at a depth of 500 feet, which is built to discharge a municipal or industrial effluent into an ocean or other body of water, may start at the shore with a 300% offset weighting to counter strong wave induced forces. After acquiring a distance of 1,000 feet from the shore and a depth of 75 feet the offset weight may be reduced to 110%. At 3,000 feet from the shore the pipeline depth is 200 feet and the offset weight may be further reduced to 100%. A change in pipeline elevation, necessitated because of a rise in the sea bed starting at 5,000 feet from the shore, which now exposes the pipeline to littoral or wave induced currents, may now require the offset weight to be increased to 150% for a 1,500 foot section of the pipeline. The remainder of the pipeline, after the 1500 foot section over the rise in sea bed elevation, may now have the offset weight reduced to 100% for the remaining 3,500 feet.
The invention allows the designers and installers of pipelines the ability to change the offset ballast weight of any section of the pipeline as dictated by ocean currents or sub sea terrain. This change in ballast weight is accomplished by increasing or decreasing the diameter of the ballast tubes in the specific area in question.
It is common practice for installers of pipelines, such as those manufactured from HDPE, to construct the pipeline on land, adjacent to the edge of the body of water where it is to be installed, complete with all of its attached ballast weights and if required, for pipelines with offset weight designed at or exceeding 100%, auxiliary buoyancy vessels. Depending upon the total length and size of the pipeline as well as the amount of ballast weight attached, it is possible to launch the complete hermetically sealed pipeline into the water to float upon its own inherent buoyancy or the combination of its own buoyancy supplemented by the auxiliary buoyancy vessels.
It is often not practical to launch pipelines of great size and length in one piece, because of the sheer bulk of the total aggregate weight of the pipeline and its components, and installers of such pipelines may choose to fabricate the pipeline in shorter more easily managed sections. The shorter sections are subsequently launched individually and can be joined together from a barge equipped for this purpose or the sections can be joined together as they enter the water one behind the other. Regardless of the method of launching the pipeline, the same basic principles apply, that is, to position the floating hermetically sealed pipeline over top of the chosen underwater pipeline corridor, and then to remove or otherwise delete the buoyancy factor to allow the pipeline to sink to the sea bed.
As previously mentioned, offset weight requirements in excess of 100% will generally require that additional buoyancy, commonly referred to as auxiliary buoyancy, in the form of vessels, floats etc., be temporarily attached to the pipeline. While laying the pipeline onto the sea bed, the auxiliary buoyancy vessels are flooded or otherwise removed from the pipeline. Auxiliary buoyancy vessels generally are designed to be filled with air and therefore must meet the structural requirements for pressure vessels due to the pressures at depth that they will be exposed to. Auxiliary buoyancy vessels may not be practical for pipelines laid at great depths because of the significant expense to manufacture them. As previously mentioned, an alternative to attaching auxiliary buoyancy is to delete some of the ballast weight from the pipeline prior to installation. The ballast weight deleted is installed after the placement of the pipeline on the sea bed. This can be accomplished by lowering the ballast weights from the water surface and placing the ballast weights on the pipeline with the use of divers or remotely operated vehicles. This method can become economically impractical, depending upon the length of the pipeline and the depth of the area requiring post ballasting.
xe2x80x9cSxe2x80x9d bend sinking is a commonly employed method of positioning synthetic pipelines, complete with ballast weighting attached, to the sea bed. xe2x80x9cSxe2x80x9d bend sinking is accomplished by introducing water at one end of a floating pipeline while simultaneously venting air from the opposite end. As the water is introduced the pipeline loses its inherent buoyancy and the end of the pipeline, where the water is being introduced, sinks to the sea bed. The remainder of the pipeline is floating on and between the water surface, until such time as the water being introduced propagates further along the pipeline causing the further reduction of buoyancy and allowing the sinking to continue until the entire pipeline rests upon the sea bed. During the sinking process, the portion of the pipeline from the last point touching the sea bed to the portion floating on the surface forms the approximate shape of an xe2x80x9cSxe2x80x9d. During the sinking process the pipeline is subjected to bending stresses throughout the xe2x80x9cSxe2x80x9d which must be controlled by applying axial tension to the pipeline to limit the amount of curvature in the pipeline. Failure to minimize the bending stress in the pipeline by the application of axial tension will result in the buckling of the pipeline. The amount of tension applied is dependent upon the composition of the material the pipeline is manufactured of, the size of the pipeline, the underwater terrain, the depth of water, and the total net installation weight, that is the weight of the flooded pipeline submerged in water minus any auxiliary buoyancy vessels attached.
Pipelines with offset weighting approaching 100%, and laid in deep water may require enormous amounts of tension to be maintained during their sinking as a means of maintaining the pipeline curvature within the minimum-bending radius as specified by the manufacturer of the pipe. If the proper tension is not maintained throughout the sinking process serious damage to the pipeline will result. Generally, large tugboats or winches are employed to achieve these tension requirements.
Applying large amounts of axial tension, especially in the case of thin wall synthetic pipelines, may not be practical as the tension requirements may exceed the tensile strength of the pipeline composition. Applying great amounts of axial tension can invoke significant costs and is often not practical from an economical point of view.
The use of auxiliary buoyancy vessels, in the form of a continuous pipe or tube, attached to pipelines is well established. The method of filling containers with a substance possessing a relative density greater than or less than water, to achieve ballast weight or buoyancy, is also common knowledge. Prior art in this field, as shown by Lamy, U.S. Pat. No. 4,062,198, utilizes a ballast pipe of the same length as that of the pipeline being installed, attached to the pipeline as a means of achieving auxiliary buoyancy over a specific section of pipeline, in such a way as to allow a reduction in apparent weight for a specific section of the pipeline, while the pipeline is being installed by means of dragging the pipeline along the sea bed. Lamy, U.S. Pat. No. 4,052,862 relates to methods of installing steel pipelines across rivers or bodies of water and entails laying steel pipelines encased within another steel pipe as a means of facilitating the installation and protecting the pipeline from corrosion or damage. Lamy, U.S. Pat. No. 4,052,862 also describes a method of filling the annulus created between the two pipes with mediums of various relative densities as a means of protecting the pipeline from corrosive damage. Neither of the aforementioned methods considers the ballast weight considerations of pipelines manufactured from synthetic materials laid in deep waters. Lamy""s U.S. Pat. No. 4,052,862 method would not be practical for synthetic pipelines, and in fact would result in damaging synthetic pipelines enclosed within a ridged container such as a steel pipe, having the annulus filled with a substance with a relative density greater than water, because of the hydrostatic head pressure exerted by the mixture around the synthetic pipe. The hydrostatic pressure exerted on synthetic pipelines surrounded by a mixture consisting of a substance with a relative density greater than water would cause compression of the internal synthetic pipeline resulting in damage to the pipe wall.
Prior art in this field, as shown by Conner, U.S. Pat. No. 3,467,013 relates to a method of transporting water great distances with the use of a membrane wall pipeline secured to the sea floor with mechanical anchors and does not concern itself with the methods of installing pipelines underwater. Conner makes reference to a longitudinally extending chamber, which appears to be a continuous tube,-the function of which is to provide a small amount of ballast weight to his system. Conner makes reference to the chamber being adapted to receive mud from the sea bed, but does not provide any supporting detail of how, or at what point, the mud would be placed within. Conner""s reference to using mud as ballast does not offer any insight or advantage to someone skilled in the art of pipeline installation. None of the above methods make any allowance for the increase or decrease in the final offset weighting to be achieved at specific sections of the pipeline, nor do they solve the unique problems associated with the installation of synthetic pipelines in deep water.
This invention is an improvement over existing methods and has many advantages over the traditional methods of installing and ballasting underwater pipelines. The invention is particularly well suited for pipelines manufactured from synthetic materials, such as HDPE, but the invention may also be applied to pipelines manufactured from metal or other compounds. Because synthetic pipelines lack the tensile and compressive strength commonly associated with iron alloy pipelines they are prone to become damaged during their installation. The risk of damage increases with the amount of ballast weight applied and the depth of water where the pipeline is to be installed. The invention allows the final ballast weight to be achieved after the pipeline, which has had its weight reduced to the minimum amount necessary for sinking purposes, has been positioned on the sea bed. After positioning the lightly weighted pipeline on the sea bed, the ballast tubes attached to the pipeline are filled, by injection or other means, with a mixture consisting of a substance which possesses a relative density that is greater than water. The invention also makes provisions for any ratio of final ballast weight to be applied to the pipeline after its final positioning on the sea bed, while still retaining the obvious benefits of low weight during the launch and sinking phases. The invention makes further provisions to allow for an increase or decrease of the amount of final ballast weight applied to specific sections along the length of the pipeline, by making changes to the diameter of the ballast tubes as required at the specific sections.
It is the purpose of this invention to allow the designers and installers of synthetic pipelines the ability to reduce the amount of axial tension required during the sinking process, by reducing the amount of weight affecting the pipeline during the sinking phase of the installation. The invention allows for any percentage of offset weighting to be achieved, while minimizing the axial tension requirements during the sinking phase. For example, a synthetic pipeline with a final offset weight requirement of 200% can be installed in much the same fashion as a synthetic pipeline with a final offset weight requirement of 5%. This method will also mitigate the risk of damage to the pipeline, which may be a consequence of insufficient axial tension applied during the sinking process.
It is also the purpose of this invention to reduce the weight of synthetic pipelines, which are prefabricated on shore, to assist with the launching of these pipelines into the water.
It is a further purpose of this invention to reduce the risk of damage to pipeline ballast systems with the elimination of the probable causes and effects of corrosion, that is, to use almost exclusively, synthetic materials which are largely unaffected by exposure to water, either fresh or salt.
Ultimately, the invention will allow the designers and installers of underwater pipelines an alternative installation method and will allow synthetic pipelines to be installed in situations that have previously been considered as impractical.